Development of long lifespan high-energy aqueous organic||iodine rechargeable batteries

Rechargeable aqueous metal||I2 electrochemical energy storage systems are a cost-effective alternative to conventional transition-metal-based batteries for grid energy storage. However, the growth of unfavorable metallic deposition and the irreversible formation of electrochemically inactive by-products at the negative electrode during cycling hinder their development. To circumvent these drawbacks, herein we propose 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) as negative electrode active material and a saturated mixed KCl/I2 aqueous electrolyte solution. The use of these components allows for exploiting two sequential reversible electrochemical reactions in a single cell. Indeed, when they are tested in combination with an active carbon-enveloped I2 electrode in a glass cell configuration, we report an initial specific discharge capacity of 900 mAh g−1 (electrode mass of iodine only) and an average cell discharge voltage of 1.25 V at 40 A g−1 and 25\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}±1 °C. Finally, we also report the assembly and testing of a PTCDI|KCl-I2|carbon paper multilayer pouch cell prototype with a discharge capacity retention of about 70% after 900 cycles at 80 mA and 25\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}±1 °C.

In summary: Please reorganise the paper so that the readers are being triggered, and not to have figuring out themselves where the relevant information has been placed. This too, includes selecting the important figures -which one to put in the body of the paper, and which in the supporting information. With respect to data analysis, reanalysing and deconvolution of the CV curves would be essential to better calculate the fractions of the capacitive-controlled vs the diffusive-controlled behaviour.
Reviewer #3 (Remarks to the Author): The cell voltage of the proposed organic cathode//I2 cascade battery is 2.5 V, which is well above the thermodynamic potential of the water splitting. The authors provided the LSV data to support the there is no water splitting. But it is not convincing.
There is no mention of percentage of specific capacity retention against the theoretical specific capacity of the cell. Figure 2F, no explanation for linear decrease in specific capacity with cycle number.
There was no formation of I3-, as supported from the UV-visible spectra, XPS, Raman and FTIR. What could be the possible reason? Line no. 185-189, the explanation is repeated.
Can author provides the details about specific capacity calculations?
The quantitative adsorption of I2 on activated carbon is not mentioned.
Can author provide the kinetic details of the cascade reactions? Figure 2C and 2D, CV and alpha graph, it is not clear about the practical capacity of the battery. The high capacity was due to the dip discharging. Please explain.  Also it is showing two cathodic peaks after 40th and 90th scan. The explanation provided in line no. 149 is insufficient. Figure S10, CV shows substantial increase in ohmic resistance of the PTCDI electrode after 3rd, 4th and 5th scan. Explanation is not provided.

Answer:
We are grateful to the reviewer for raising the important points. Following this reviewer's suggestion, we have evaluated the performance of the full cell at low current density (1 A g −1 ). The results revealed that the cell was unable to be charged to 2.4 V but only fluctuated in the vicinity of 1.8 V. This is consistent with the previous results (Chem. Eur. J. 2020, 26, 17559-17566), showing that PTCDI electrode is not suitable for working at the low current density. Such phenomenon is common for organic electrodes, which is mainly attributed to the irreversible side reactions induced at a low current density and the limited intrinsic conductivity of organic electrode materials (Chem. Eur. J. 2020, 26, 17559-17566;Nat. Energy 2019, 4, 495-503;Nat. Commun. 2021, 12, 2400Nat. Sustain. 2022, 5, 225-234;ACS Nano 2021, 15, 1077-1085. In this regard, a high current density is necessary to reach the required voltage and to prevent the irreversible side reactions. The results have been added in Supplementary Following this reviewer's suggestion, we have also measured the self-discharge rate of the PTCDI||I2 full cell after charging it to 2.4 V at 40 A g −1 . In fact, self-discharge behavior is common for the aqueous iodine cathode battery systems. The self-discharge is originated from the dissolubility of iodine and iodine species in the aqueous environment of the batteries (Energy Environ. Sci. 2021, 14, 407-413;Angew. Chem. Int. Ed. 2021, 60, 12636-12647;Nat. Commun. 2021, 12, 170;Angew. Chem. Int. Ed. 2022, 61, e202113576;Adv. Mater. 2021, 33, 4 / 19 2006897;Nano Lett. 2015, 15, 5982-5987;ACS Energy Lett. 2017, 2, 2674-2680. This means that regardless of which kind of iodine cathode material is utilized, iodine species will inevitably release into the aqueous electrolyte and trigger a series of complexation reactions, thus leading to the self-discharge behavior. To better evaluate the self-discharge rate of the full cell (PTCDI||I2) of this work, we constructed a Zn||I2@AC full cell (a conventional aqueous iodine full cell that has been reported extensively, e.g. Energy Environ. Sci. 2021, 14, 407-413;Angew. Chem. Int. Ed. 2021, 60, 12636-12647) in mixed electrolyte (1 M KCl and 1M ZnCl2) and evaluated its self-discharge performance after charging it to 1.8 V at 40 A g −1 , as a control group. As shown in Supplementary Figure 12, for our PTCDI||I2 full cell, a voltage drop of 0.4 V needs 78.2 s; in contrast, for the Zn||I2 full cell, the time required for the same voltage drop is as less as 27.6 s. Clearly, the PTCDI||I2@AC full cell has a much lower self-discharge rate than that of the conventional Zn||I2@AC full cell.
The results have been added in Supplementary Figure 12 and described/discussed accordingly in the revised manuscript (Page 9-10, Line 215-228) as follows: "Originated from the dissolubility of iodine and iodine species in the aqueous environment of the batteries, self-discharge behavior is common for the aqueous iodine-cathode battery systems. [3][4][5][6] How to reduce the self-discharge rate effectively has been an intriguing but challenging issue. In the cases of the conventional aqueous metal||I2 battery systems, iodine anionic species can diffuse into the vicinity of the metal anode and induce the formation of electrochemically inactive complexes, resulting in the irreversible loss of iodine elements and severe self-discharge behaviour. [3][4][5][6] Encouragingly, the anode material adopted in this work was PTCDI, an organic compound featuring intrinsic inertness to various iodine anionic species. Iodine anionic species were unable to react with the PTCDI anode to form electrochemically inactive complexes. As a result, the PTCDI||I2 full cell in this work displayed a much lower self-discharge rate than that of the conventional Zn||I2 full cell (Supplementary Figure 12). Specifically, the time required for a voltage drop of 0.4 V extended from 27.6 s for the latter to 78.2 s. That is, the self-discharge rate of the PTCDI||I2 full cell was reduced to 35.3% as that of the Zn||I2 full cell."

Reviewer #2 (Remarks to the Author):
Comments: This paper describes a very interesting concept of new type of electrochemical cell based on cheap materials. The authors were able to come up with explanations and simple models to analyse and understand the measured data.
In that respect they used a number of relevant analytical tools to proof the behaviour and existence of the electro-active species.
Answer: We highly appreciate the reviewer for raising the constructive comments and positive remarks on our manuscript. Following this reviewer's suggestions, we have reorganized the figures/main text and added new results/discussion based on the additional experiments in the revised manuscript and SI. Please refer to our point-by-point response below.
Unfortunately, the paper is quite chaotic, and actually it is way too difficult to read -that is why it took me a little ii) The schematic illustration of the symmetric cell originally located in SI has been placed in the revised manuscript as Figure 4d and Figure 4e.
iii) The contribution ratio of the capacitive effect of the cascade cell has been placed in the revised manuscript as Figure 4j.

iv)
We have added a simple overall schematic illustration of the cascade cell as Figure 1d.
v) The electrochemical steps of the PTCDI||I2 single cell and cascade cell have been added to Page 5 (Line 123-127) and Page 6 (Line 145-157) in the revised manuscript as follows:

Answer:
We are very grateful to the reviewer for raising these constructive comments. Following this reviewer's suggestion, we first carried out additional measurements on the CV curve of the PTCDI electrode at various scan rates ranging from 5 mV s −1 to 45 mV s −1 in a three-electrode system (Supplementary Figure 3c). To determine how For further determination of the capacitive contribution, the current density (i) at a fixed potential can be divided into the capacitance-dominated contribution (k1v) and the diffusion effect (k2v 1/2 ), and the relationship can be expressed as follows: [15][16][17] As a typical example, at a scan rate of 15 mV s −1 , the capacitance-controlled contribution approached 69.0%.

Moreover, it increased with increasing scan rate
---In line 122 of the supporting information, you gave a reaction with K + , but the product formed has also a proton accepted (middle product) -was wondering whether this is a typo or are protons indeed involved in this reaction?
Answer: We thank the reviewer for pointing out this typo. We have revised the schematics of the redox mechanism for reversible K + storage reaction of the PTCDI electrode in Supplementary Figure 5, and interpreted that the redox mechanism of the PTCDI electrode in the revised SI (Page 8, Line 117-119) as follows: "By combining the aforementioned CV results and actual capacity delivered, the redox mechanism of the PTCDI electrode can be expressed as a stepwise enolization reaction in terms of reversible stepwise intercalation of K + . 1, 4,

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Supplementary Figure 5. Schematics of the redox mechanism for the reversible K + storage reaction of the PTCDI electrode.
Moreover, we have added the CV curves of the PTCDI electrode measured in a dilute HCl electrolyte (pH ≈ 5) in Supplementary Figure 3b and addressed accordingly in the revised SI (Page 5, Line 65-67) as follows: "As presented in Supplementary Figure 3b, no redox peaks were observed, verifying that protons had no effect on the charge/discharge process of the PTCDI electrode." --- In Fig 2f,  Answer: We thank the reviewer for raising this concern. As the theoretical specific capacity of I2 (I − /I 0 /I + ) is 422 mAh g −1 , 1 C for I2 is 422 mA g −1 (Energy Environ. Sci. 2021, 14, 407-413;Nat. Commun. 2021, 12, 170;Angew. Chem. Int. Ed. 2022, 61, e202113576). Regarding the results in Figure 2f, the current density applied was 40 A g −1 , corresponding to 94.8 C. Consequently, it took 1890 hours, i.e. 79 days, in total to complete the 90000 cycles.
Following this reviewer's suggestion, we have added the corresponding C-rate value together with the current density in the revised manuscript (Page 4, Line 86-87; Page 9, Line 197-198) as follows: "an ultralong lifespan (92000 cycles at 40 A g −1 (94.8 C) and an extremely high rate tolerance (104 mAh g −1 at 160 A g −1 )," "Remarkably, a discharge capacity of 154 mAh g −1 was achieved after 92000 cycles at 40 A g −1 (94.8 C) (Figure 2f)." --- on additional experiments together with the analysis of the kinetic behaviors for the PTCDI electrode and the PTCDI||I2 full cell in the revised manuscript and SI. Thanks to the re-organization and revisions, the readability and quality of this work have been improved substantially.

Reviewer #3 (Remarks to the Author):
Comments: The cell voltage of the proposed organic cathode||I2 cascade battery is 2.5 V, which is well above the thermodynamic potential of the water splitting. The authors provided the LSV data to support the there is no water splitting. But it is not convincing.

Answer:
We thank the reviewer for raising this concern. First, the LSV measurement has been widely used to Following this reviewer's suggestion, to further ascertain that there is no water splitting, we have monitored in-situ gas pressure variation during the CV measurements of the cascade cell at the voltage ≤2.5 V. The results have been added in Supplementary Figure 19 and addressed accordingly in the revised manuscript (Page 17, Line 379-382; Page 23, Line 511-513) as follows: "In addition, no fluctuation in gas pressure was detected during the whole process of the cascade cell, indicating that no H2 or O2 evolution reaction occurred (Supplementary Figure 19). 64  ---There is no mention of percentage of specific capacity retention against the theoretical specific capacity of the cell. Figure 2F, no explanation for linear decrease in specific capacity with cycle number.

Answer:
We thank the reviewer for raising the valuable point. Following this reviewer's suggestion, we have added the percentage of specific capacity retention against the theoretical specific capacity of the cell in the revised manuscript (Page 9, Line 198-201) as follows: "The percentage of specific capacity retention against the theoretical (422 mAh g −1 ) and initial (363.4 mAh g −1 ) specific capacity of the cell after 92000 cycles were calculated to be 36.5% and 42.4%, respectively, delivering outstanding decay ratios of 0.7% and 0.6% per thousand cycles, respectively."

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The linear decrease of the specific capacity with the cycle number is mainly attributed to the solubility of iodine molecules and iodine anionic species derived from the I2@AC cathode, which is a common phenomenon for the aqueous iodine-cathode battery systems (Energy Environ. Sci. 2021, 14, 407-413;Nat. Commun. 2021, 12, 170;Adv. Mater. 2021, 33, 2006897). Fortunately, the PTCDI compound used in this work possesses a large π-conjugated structure with strong π-π interactions and intermolecular hydrogen bonding, which can endow the anode excellent structural stability during the repeated charge/discharge process (Nat. Sustain. 2022, 5 Answer: We thank the reviewer for raising this concern. In fact, in the reference mentioned by the reviewer (https://doi.org/10.1002/chem.202003624), on the contrary to raising the concern about the oxidative stability of PTCDI, the authors confirmed the long-term cycling stability of PTCDI at high oxidation potential: "Organic cathode materials are known for their poor cycle life performance, due to their structural instability particularly at high oxidation potentials and their tendency to dissolve in organic battery electrolytes. In contrast, the long-term cycling stability of PTCDI is exceptionally enhanced, possibly due to the larger π-conjugated structure and therefore Answer: We thank the reviewer for raising this question. The absence of I3 − is mainly attributed to the presence of K + ions in aqueous electrolyte, which is beneficial to a direct conversion between I − and I 0 . According to our DFT 13 / 19 calculations, the lowest value of Gibbs free energy change (∆G) was observed when the I − /I 0 conversion chemistry occurred in the K + environment (Figure 1a). In this way, the aqueous electrolyte containing K + holds the maximum potential to accelerate the I − /I 0 conversion, superior to other cations (Energy Environ. Sci. 2021, 14, 407-413;Angew. Chem. Int. Ed. 2021, 60, 3791-3798). In the present work, saturated KCl solution was utilized as electrolyte for the PTCDI||I2 full cell, which facilitated the fast reaction rate of the I − /I 0 conversion. Following this reviewer's suggestion, we have carried out additional CV measurements of the I2@AC electrode in a saturated KCl electrolyte and added the results in Supplementary Figure 13 and addressed them in the revised SI (Page 19, Line 255-259) and manuscript (Page 12, Line 266-269) as follows: SI: "As shown in Supplementary Figure 13, the CV of the I2@AC electrode in a saturated KCl electrolyte was tested in a three-electrode system. Only a pair of redox peaks corresponding to I − /I 0 was observed, and no other redox peaks corresponding to I − /I3 − were detected, confirming a direct conversion between I − and I 0 . 13,31,40 These results are consistent with the DFT result in Figure 1a. 13,25,30,31,41 "  --- Figure 2C and 2D, CV and alpha graph, it is not clear about the practical capacity of the battery. The high capacity was due to the dip discharging. Please explain.
Answer: We thank the reviewer for raising this important comment. To make it more specific, the y-axis has been adjusted from "current" to "specific current" in Figure 2c.  Supplementary Figure 9, a discharge capacity of 300 mAh g −1 was delivered, which still far exceeded that of all reported aqueous iodine-cathode batteries at such a high current density. 13,25,[29][30][31][32][33][34] An additional discharge capacity of 24 mAh g −1 was delivered when the cut-off voltage of the full cell was set to 0, which only accounted for 7.4% of the total capacity (324 mAh g −1 in Figure 2d). Therefore, the cut-off voltage of the full cell was not the main reason for the high capacity delivered. Instead, a slightly higher capacity output was obtained if the cut-off voltage was set to 0, and the same method has also been reported in previous works. 2,8,22 " Main text: "The high capacity mainly originated from the two-electron transfer reaction of the I2 cathode and was not greatly affected by the cut-off voltage of the cell (Supplementary Figure 9)."  (Supplementary Figure 3f). The high capacitive contribution of the PTCD electrode is a common phenomenon and has been widely reported in previous works. 1,3,4,8,[19][20][21][22] The high capacitive contribution means that the kinetic behaviour of the electrode is mainly dominated by the capacitive effect, and the transport of K + ions is not the rate-limiting factor. 21 In other words, the transport of K + ions is fast enough under high operational current densities. In addition, the capacitive effect renders more charge transfer than volume lattice diffusion and thus can help to retain the capacity at high operational current densities. 23 --- Figure S10, CV shows substantial increase in ohmic resistance of the PTCDI electrode after 3rd, 4th and 5th scan.
Explanation is not provided.

Answer:
We thank the reviewer for raising this comment. Following this reviewer's suggestion, we have carried out additional EIS measurements of the PTCDI electrode after different scan cycles to determine the variation in the ohmic resistance. The results have been added in Supplementary Figure 6 and addressed accordingly in the revised SI (Page 9, Line 127-139) as follows: "As presented in Supplementary Figure 6a, the CV curves of the PTCDI electrode in the saturated KCl + I2(aq) mixed electrolyte were identical to those in the pure saturated KCl electrolyte (Supplementary Figure 3a) Answer: We highly appreciate the reviewer for positive remarks on our manuscript.

Comments:
The authors have answered the concern raised by reviewers but till some more explanation is necessary The question raised by the second reviewer regarding the ' -In Fig 2f, Figure 2f, the current density applied was 40 A g −1 , corresponding to 94.8 C, which means that a full cycle took 0.021 hour. Consequently, it took 1890 hours, i.e. 79 days, in total to complete the 90000 cycles. Following the reviewer's suggestion, we have added the corresponding C-rate value together with the current density to the revised manuscript ( Moreover, to prevent the evaporation of electrolyte during the long cycle test (over two months), the full cell was hermetically sealed by laboratory paper film and no crystallization of electrolyte was observed. The related information has been added to the revised manuscript (Page 23, Line 491-494) as follows: Main text: "The full cell was hermetically sealed by laboratory paper film (PM996,Bemis,America) to guarantee that electrodes were immersed into the electrolyte during the whole test and no crystallization of electrolyte was observed." Regarding the question of high capacity and dip discharge, Figure 2C shows Main text: "When the cut-off voltage of the cell was set to 0.3 V, a discharge capacity of 300 mAh g −1 with the coulombic efficiency of 82% was delivered (Supplementary Figure 9). Hence, the cutoff voltage of the full cell had the effect on the specific capacity and coulombic efficiency to some extent. Considering that a slightly higher capacity output was obtained, the cut-off voltage of the full cell was set to 0 in this work." Considering that the discharge curve has multiple discharge platforms, herein, the average voltage of 1.26 V was adopted to evaluate the practical voltage of the full cell. This voltage is far

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higher than the required voltage of some practical applications such as potato clock etc. We have revised the manuscript (Page 8, Line 183-186) as follows: Main text: "The typical galvanostatic charge/discharge (GCD) curve displayed the maximal plateau at 1.90 V and average voltage of 1.26 V during the discharge process, and a discharge capacity of 324 mAh g −1 with the coulombic efficiency of 69% was delivered at 40 A g −1 (94.8 C) (Figure 2d)."